---
title: "CVE-2026-25521: locutus is vulnerable to Prototype Pollution"
url: https://erroneo.us/security/cve/CVE-2026-25521/
description: "CVE-2026-25521 (locutus (npm)): Locutus brings stdlibs of other programming languages to JavaScript for educational purposes. In versions from 2.0.12 to…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-02-04
date_modified: 2026-10-05
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-25521"
kev: false
cvss: 9.4
epss: 0.00498
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-25521: locutus is vulnerable to Prototype Pollution

**Signal:** Critical, preconditions apply. Severe if exploited, but exploitation needs conditions that may not apply to you. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.5% , percentile 42: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 42% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Needs local access needed. Lower urgency for most teams unless an attacker could already run code on the machine.
- Only applies if your dependency tree includes locutus (npm) >= 2.0.12, < 2.0.39, fixed in 2.0.39 . Check your lockfile, including transitive dependencies.

## What it is

Locutus brings stdlibs of other programming languages to JavaScript for educational purposes. In versions from 2.0.12 to before 2.0.39, a prototype pollution vulnerability exists in locutus. Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input contained a forbidden key, it is still possible to pollute Object.prototype via a crafted input using String.prototype. This issue has been patched in version 2.0.39.

## Affected packages

- npm `locutus` >= 2.0.12, < 2.0.39 → fixed: 2.0.39

## Affected products (CVE record)

- locutusjs locutus: >= 2.0.12, < 2.0.39

## Severity

- GitHub_M CVSS 4.0: 9.4 `CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`
- redhat-SADP CVSS 3.1: 9.3 `CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H`
- GitHub CVSS 4.0: 9.4 `CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.50% probability of exploitation activity in the next 30 days; percentile 42.

## Sources

- [GitHub Advisory GHSA-rxrv-835q-v5mh](https://github.com/advisories/GHSA-rxrv-835q-v5mh)
- [CVE.org record CVE-2026-25521](https://www.cve.org/CVERecord?id=CVE-2026-25521)
- [NVD entry CVE-2026-25521](https://nvd.nist.gov/vuln/detail/CVE-2026-25521)
- [github.com](https://github.com/locutusjs/locutus/security/advisories/GHSA-rxrv-835q-v5mh)
- [github.com](https://github.com/locutusjs/locutus/commit/042af9ca7fde2ff599120783e720a17f335bb01c)
- [access.redhat.com](https://access.redhat.com/security/cve/CVE-2026-25521)
- [RHBZ#2436950](https://bugzilla.redhat.com/show_bug.cgi?id=2436950)
- [security.access.redhat.com](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-25521.json)

---
Source page: https://erroneo.us/security/cve/CVE-2026-25521/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt